fix: convert the displayed calendar date in toHijri for hijri-core's UTC-day contract

moment.fn.toHijri now passes new Date(Date.UTC(this.year(), this.month(), this.date()))
to hijri-core instead of the raw instant (this.toDate()). This converts the calendar
date the moment instance displays, respecting utc() mode, rather than the underlying
millisecond value — eliminating wrong-Hijri-day results around UTC-midnight for hosts
east or west of UTC.

Lock-step with hijri-core fix/utc-day-boundary (commit 3419378). fromHijri path
was already correct; its comment updated for clarity.

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- `.toHijri()` now converts the calendar date the moment instance displays (year/month/day
+  components, respecting `.utc()` mode) rather than passing the raw instant to hijri-core.
+  This eliminates wrong-Hijri-day results around UTC-midnight for hosts east or west of UTC.
+  Lock-step with the unreleased hijri-core fix on `fix/utc-day-boundary`.
+
 ## [1.0.2] - 2026-05-30
 
 ### Changed

README.md

@@ -50,6 +50,15 @@ Pass `{ calendar: 'fcna' }` to switch from the default Umm al-Qura calendar to F
 Full API reference, format token table, and examples are in the
 [project wiki](https://github.com/acamarata/moment-hijri-plus/wiki).
 
+## Day boundaries and time zones
+
+Conversions use the calendar date the moment instance displays, not the underlying UTC
+instant. A `moment("2025-03-01")` parsed in any local timezone returns the Hijri date
+for March 1st, 2025. A moment created with `.utc()` uses its UTC components.
+
+Religious day-start at sunset is outside the scope of this package; it depends on
+location and madhab, and must be handled at the application layer.
+
 ## Note on Moment.js
 
 Moment.js is in maintenance mode. For new projects,

src/index.ts

@@ -28,8 +28,13 @@ declare module "moment" {
     /**
      * Convert this moment to a Hijri date.
      *
-     * Passes the underlying `Date` to hijri-core's `toHijri()`. The calendar engine
+     * Converts the calendar date this moment instance displays (year/month/day) to a
-     * performs a table lookup (UAQ) or astronomical calculation (FCNA).
+     * Hijri date via hijri-core's `toHijri()`. The conversion is independent of the
+     * host machine's timezone: a moment in UTC mode uses its UTC components, and a
+     * moment in local mode uses its local components. The raw instant (milliseconds
+     * since epoch) is never passed directly to the calendar engine.
+     *
+     * The calendar engine performs a table lookup (UAQ) or astronomical calculation (FCNA).
      *
      * @param options - Calendar selection. Default: `{ calendar: 'uaq' }`.
      * @returns A `HijriDate` object `{ hy, hm, hd }`, or `null` if the date is outside
@@ -149,7 +154,11 @@ function escapeLiteral(value: string): string {
  */
 function install(momentInstance: typeof moment): void {
   momentInstance.fn.toHijri = function (opts?: ConversionOptions): HijriDate | null {
-    return toHijri(this.toDate(), opts);
+    // Use the calendar date this instance displays rather than the raw instant.
+    // this.year()/.month()/.date() respect utc mode automatically, so a moment
+    // created with .utc() uses UTC components and a local moment uses local components.
+    // moment.month() is 0-based, matching Date.UTC's month parameter exactly.
+    return toHijri(new Date(Date.UTC(this.year(), this.month(), this.date())), opts);
   };
 
   momentInstance.fn.hijriYear = function (opts?: ConversionOptions): number | null {
@@ -236,8 +245,8 @@ function install(momentInstance: typeof moment): void {
     if (!greg) {
       throw new Error(`Invalid or out-of-range Hijri date: ${hy}/${hm}/${hd}`);
     }
-    // Construct from explicit year/month/day to avoid UTC-to-local timezone
+    // Construct from explicit year/month/day components so the moment displays
-    // shift when the Date object represents midnight UTC.
+    // the correct calendar date regardless of the host timezone offset.
     return momentInstance([greg.getUTCFullYear(), greg.getUTCMonth(), greg.getUTCDate()]);
   };
 }

test-cjs.cjs

@@ -61,3 +61,22 @@ describe('CJS: isValidHijri', () => {
     assert.equal(moment(new Date(2023, 2, 23, 12)).isValidHijri(), true);
   });
 });
+
+describe('CJS: UTC-day boundary (regression)', () => {
+  it('fromHijri → toHijri round-trip: 1446/9/1', () => {
+    const m = moment.fromHijri(1446, 9, 1);
+    const h = m.toHijri();
+    assert.notEqual(h, null);
+    assert.equal(h.hy, 1446);
+    assert.equal(h.hm, 9);
+    assert.equal(h.hd, 1);
+  });
+
+  it('moment("2025-03-01") toHijri => 1446/9/1 (timezone-invariant)', () => {
+    const h = moment('2025-03-01').toHijri();
+    assert.notEqual(h, null);
+    assert.equal(h.hy, 1446);
+    assert.equal(h.hm, 9);
+    assert.equal(h.hd, 1);
+  });
+});

test.mjs

@@ -110,3 +110,26 @@ describe('FCNA calendar', () => {
     assert.equal(typeof h.hd, 'number');
   });
 });
+
+describe('UTC-day boundary (regression)', () => {
+  it('fromHijri → toHijri round-trip: 1446/9/1', () => {
+    // Construct from Hijri, convert back — must be exact regardless of host TZ.
+    const m = moment.fromHijri(1446, 9, 1);
+    const h = m.toHijri();
+    assert.notEqual(h, null);
+    assert.equal(h.hy, 1446);
+    assert.equal(h.hm, 9);
+    assert.equal(h.hd, 1);
+  });
+
+  it('moment("2025-03-01") toHijri => 1446/9/1 (timezone-invariant)', () => {
+    // moment parses date-only ISO strings as LOCAL midnight.
+    // toHijri must convert the displayed calendar date (2025-03-01), not the raw
+    // instant, so the result is the same regardless of the host timezone offset.
+    const h = moment('2025-03-01').toHijri();
+    assert.notEqual(h, null);
+    assert.equal(h.hy, 1446);
+    assert.equal(h.hm, 9);
+    assert.equal(h.hd, 1);
+  });
+});